In digital communications systems, local clock signals must be phase aligned with incoming data signals so that the incoming data can be correctly interpreted. In a conventional phase alignment method, a phase difference of an incoming data signal and a local clock signal is detected. If the local clock signal lags the incoming data signal by more than one half clock period, the phase of the local clock signal is advanced. If the local clock signal lags the incoming data signal by less than one half clock period, the phase of the local clock signal is retarded. Ideally, the phase of the local clock signal is maintained approximately one half clock period behind the phase of the incoming data signal so that a timing edge of the local clock signal is kept near the center of the "data eye".
In the conventional phase alignment method, the phase of the local clock signal must be adjusted rapidly enough to keep the timing edge within the "data eye". If the timing edge reaches the edge of the "data eye" (i.e. if the timing edge is allowed to pass a data edge) a bit of the incoming data stream will either be lost or detected twice. The rate at which the local clock phase is adjusted must be high enough to prevent such errors. Unfortunately, a high rate of adjustment for the phase of the local clock signal increases phase noise in the local clock signal.